In general, a field emission device emits light using cathodoluminescence in a fluorescent layer on an anode substrate by causing electrons emitted from a field emitter on a cathode substrate to collide with the fluorescent layer. Here, the cathode substrate is disposed opposite to and spaced apart from the anode substrate by a specific distance, and the substrates are vacuum-packaged. Recently, a field emission lamp has been studied and developed as an alternative to a backlight unit for a conventional liquid crystal display (LCD), a flat light device, and a typical illumination device. In particular, the backlight unit generally includes a cold cathode fluorescent lamp (CCFL) or a light emitting diode. The CCFL backlight unit has advantages and disadvantages. The disadvantages include high manufacturing cost, environmental pollution, and nonuniform emission in, for example, a large display device.
To solve the problems, a field emission backlight unit with a relatively simple structure has been suggested. The field emission backlight unit has advantages of low manufacturing cost, mercury-free environmentally-friendly configuration, and low power consumption in comparison with a cold cathode fluorescent lamp.
As one sort of a field emission device, a conventional field emission backlight unit may be variously classified into, for example, those shown in FIGS. 1, 2 and 3.
FIG. 1 illustrates a diode-type field emission device.
Referring to FIG. 1, the diode-type field emission device, e.g., a field emission backlight unit includes an anode substrate 110, and a cathode substrate 140 disposed opposite to and spaced apart from the anode substrate 110 by a predetermined distance. An anode electrode 120 and a phosphor layer 130 are formed on the anode substrate 110 toward the cathode substrate 140. A cathode electrode 150 and a field emitter 160 are formed on the cathode substrate 140 toward the anode substrate 110.
In the field emission backlight unit having the above configuration, the field emitter 160 (e.g., carbon nanotube; CNT), which is formed on the cathode electrode 150 on the cathode substrate 140, emits elections. The electrons are induced and accelerated by a voltage applied to the anode electrode 120 on the anode substrate 110, which is disposed opposite to the cathode substrate 140 at a certain interval. A beam of electrons emitted from the field emitter 160 collides with the fluorescent layer 130 formed on the anode electrode 120, which absorbs energy of the electrons to emit a visible ray.
The diode-type field emission backlight unit can be easily manufactured because of its simple structure. However, arc discharge occurring in a free space between the cathode substrate 140 and the anode substrate 110 makes it difficult to apply a high voltage to the anode electrode 120, thus degrading fluorescence efficiency. In addition, it degrades uniformity of the electron beam emitted from the field emitter 160. Accordingly, it is difficult to attain uniform emission over the surface of the substrate including the fluorescent layer 130.
FIG. 2 illustrates a triode-type field emission device. Referring to FIG. 2, the triode-type field emission device, e.g., a field emission backlight unit includes an anode substrate 110 having an anode electrode 120 and a fluorescent layer 130, and a cathode substrate 140. A cathode electrode 150 is formed on the cathode substrate 140, and a plurality of insulators 169 are formed on the cathode substrate 140, with the cathode electrode 150 interposed between insulators 169. A field emitter 160 is formed on the cathode electrode 150, a gate electrode 180 is formed on each insulator 169, and an opening 190 exposing the field emitter 160 is formed between the gate electrodes 180.
In the above structure, electrons are induced and emitted from the field emitter 160 by a voltage applied to the gate electrode 180, which is electrically isolated from the cathode electrode 150 by the insulators 169. The emitted electrons are accelerated by a voltage applied to the anode electrode 120 to collide with the fluorescent layer 130. In principle, an amount of the electrons emitted by the field emitter 160 must depend on the cathode electrode 150 and the voltage applied to the anode electrode 120 should contribute only to the acceleration of the emitted electrons. However, since the insulators 169 are generally thinner than the opening 190 formed between the insulators 169 by a thin film process, the gate electrode 180 does not entirely block an electric field formed by the anode electrode 120. Accordingly, it is difficult to attain complete triode operation and apply a high anode voltage, as in the diode type.
FIG. 3 illustrates a lateral triode-type field emission device. Referring to FIG. 3, the triode-type field emission device, e.g., a field emission backlight unit includes an anode substrate 110 having an anode electrode 120 and a fluorescent layer 130, and a cathode substrate 140. A cathode electrode 150 and a gate electrode 180 are formed on the cathode substrate 140 and disposed adjacent to each other. Field emitters 160 are formed on the cathode electrode 150 and the gate electrode 180, respectively. The cathode electrode 150 or the gate electrode 180 function as a cathode electrode or a gate electrode according to a voltage difference between the two electrodes 150 and 180. Electrons emitted from the field emitters 160, which are formed on one surface of each of the electrodes 150 and 180, are accelerated by the anode electrode 120 to collide with the fluorescent layer 130. This lateral triode-type structure can be easily manufactured in comparison with the typical triode-type structure shown in FIG. 2 and driven by an AC signal, thereby improving an emission characteristic, but is fundamentally susceptible to a high anode voltage.
In general, a fluorescent substance used in a high-voltage cathode ray tube (CRT), when colliding with electrons accelerated by a high voltage, exhibits a proper emission characteristic. According to conventional knowledge, a phosphor exhibiting a good characteristic in a low-voltage condition does not exist. Accordingly, to obtain a proper characteristic of a high-voltage phosphor, a sufficiently high voltage needs to be applied to the anode electrode 120. However, in the case of the typical triode-type field emission backlight unit of FIG. 2, the gate insulators 169 are thinner than the opening 190, and when a higher anode voltage is applied, the field emitter 160 is damaged by arc discharge and a perfect triode operation is not attained so that electron emission does not depend on only the gate voltage but also the anode voltage.
FIGS. 4a and 4b are plan views of the typical triode-type field emission device of FIG. 2. Referring to FIGS. 4a and 4b, the gate electrode 180 having a different opening 190 surrounds the field emitter 160. In this case, the electron beam emitted by the voltage applied to the gate electrode 180 is directly induced toward the anode electrode 120 (see FIG. 2). To fill a space between the adjacent field emitters 160 that the electron beam does not reach, the number of unit openings 190 formed for electron beam emission or the distance between the anode substrate 110 and the cathode substrate 140 must increase to spread the electron beam. However, the increased number of the openings 190 or the field emitters 160 makes it difficult to attain process yield and uniform arrangement of the emitters. Furthermore, because the distance between the anode substrate 110 and the cathode substrate 140 cannot increase indefinitely due to structural limitations, it is difficult to obtain a highly uniform emission characteristic.